1. Field of the Invention
The present invention is directed to a method for manufacturing a carbon sheet and in particular to a method for manufacturing a carbon sheet which is used as a raw material of an electrode for a fuel cell. The present application is based upon Japanese patent applications 10-240743, 10-270745 and 11-213508, all of which are hereby incorporated by reference into the present application.
2. Description of the Related Art
In general, a fuel cell includes many cells arranged in a layered configuration. Each cell is a membrane electrode assembly (hereinafter referred to simply as an `MEA`) and includes an ion exchange membrane interposed between two porous, electrically conductive substrates, i.e. a fuel electrode and an oxygen electrode.
At the fuel electrode, when being provided with a fuel gas which contains therein as its main component hydrogen, the following reaction occurs. EQU 2H.sub.2.fwdarw.4H.sup.- +4e.sup.-
The resultant hydrogen ions (4H.sup.-) pass through the ion exchange membrane to arrive at the oxygen electrode to which oxygen is provided. The electrons (4e.sup.-) produced by the foregoing reaction arrive at the oxygen electrode by way of a connector wire. As a result, at the oxygen electrode, the following reaction occurs.
4H.sup.- +4e.sup.- +O.sub.2.fwdarw.2H.sub.2 O
Thus, this fuel cell functions as a battery because it produces electrons to generate an electric current. Due to the fact that the foregoing electric current generation is based on the reverse mode of the electrolyzing water, which exhausts no substances except for water in liquid phase, such an electric current generator has become attractive as a clean or nonpolluting generator.
In order to reduce air pollution, the widespread use of electrically operated vehicles which mount fuel cells is desired. However, this desire has not been attained. The main reason is that the production cost of the fuel cells, per se, is too high. In light of the fact that most of the production cost of the fuel cells is represented by the production cost of the electrodes, it is very important to realize a cost reduction for the electrodes.
A conventional fuel or oxygen electrode is in the form of a catalyst carrying carbon sheet which is obtained by applying a catalyst on a water repellant treated carbon paper. Such a carbon sheet must be high in gas permeability, excellent in electric conductivity, high in water repellence, thermally and chemically stable, and low in production cost.
The production of water at the oxygen electrode may cause a problem if too much water accumulates in an oxidant flow path. If the oxidant stream becomes saturated, two phase flow may occur. That is, the oxidant flow stream may contain both water vapor and liquid droplets. Liquid water in the oxidant flow can flood the porous electrode and obstruct the oxidant from reaching the catalyst on the oxygen electrode.
In order to solve such a `flood` problem, it is effective, as well known, to quickly discharge the generated water by keeping the electrode wet. On the basis of this concept, Japanese Patent Laid-Open Print No. Hei. 7(1995)-13 0374 provides an electrode, in the form of a catalyst carried carbon sheet, for a solid high molecular electrolysis type fuel cell. This electrode is obtained by the steps of immersing a commercially available carbon paper having a porosity of 80% in a liquid in which polytetrafluoroethylene family particles are dispersed, removing the carbon paper from the liquid, and baking the carbon paper.
However, in the commercially available carbon paper, carbon fibers, after being coupled with a thermosetting resin, are hot pressed under ambient conditions at a temperature of not less than 1000.degree. Celsius, which results in high production costs. Thus, a fuel cell employing such an electrode becomes high in cost and is difficult to be put into practical use.
Alternatively, in Japanese Patent Laid-Open Print No. Hei. 10(1998)-270052, an outer surface of a substrate of an electrode is coated with a water-repellent material by exposing the substrate to fluorocarbon gas which is activated by a plasma treatment. However, employing such a method requires specially designed, large scale equipment or a similar facility, which means that such an electrode is not suitable for mass production and is not acceptable from a cost viewpoint.